1. Field of the Invention
The present invention relates to a vehicular height control device for use in, for example, an automobile, a motorcycle, or an industrial vehicle.
2. Prior Art
As a coventional vehicular height control device, for example, there has been provided such a self-pumping shock absorber as shown in FIG. 2. A brief description will now be given about this conventional device.
In FIG. 2, reference numeral 15 denotes a cylinder disposed within a shell member 16 which is secured to the body of a vehicle. Numeral 11 denotes a piston rod secured to a wheel and having a piston 3 which is adapted to slide within the cylinder 15. Numeral 10 denotes a pump cylinder disposed within the piston rod 11. The pump cylinder 10 defines an oil passage 17 between itself and the piston rod 11, the oil passage 17 being in communication with upper and lower oil chambers 13,13 of a high pressure partitioned by the piston 3.
Numeral 9 denotes a pump rod held at one end thereof by a bottom (inner surface of the upper end) of the shell member 16. The opposite end of the pump rod 9 is inserted into the pump cylinder 10. Halfway through the pump rod 9 is formed a control orifice 2 which is open to the upper oil chamber 13 and which functions to lower the vehicular height.
Numeral 1 denotes an oil reservoir formed in the interior of the shell member 16. The oil reservoir 1 is in communication with a pump chamber 12 through the pump rod 9 and an inlet valve 6, the pump chamber 12 being formed within the lower portion of the pump cylinder 10.
Numeral 7 denotes an outlet valve disposed between the pump chamber 12 located in the lower portion of the pump cylinder 10 and the oil passage 17. Numeral 4 denotes a diaphragm which provides a partition between a high pressure chamber 18 and a gas chamber 5 in the interior of the shell member 16, the high pressure chamber 18 being in communication with the upper oil chamber 13.
Numeral 8 denotes a recess formed in part of the outer periphery of the pump rod 9 at a certain length in an axial direction thereof. The recess 8 functions so as to provide communication between the upper oil chamber 13 and the pump chamber 12 when its upper portion is positioned above the pump cylinder 10.
Next, the operation of this conventional shock absorber will be described. When the vehicular height becomes lower due to an increase in load on the vehicle, the whole of the shock absorber as a vehicular height control device undergoes a compressive force and is compressed thereby. At this time, the control orifice 2 and the recess 8 are positioned within the pump cylinder 10.
In this state, when an exciting force is exerted on the shock absorber during vehicular running, the oil present in the oil reservoir 1 flows, in an expansion stroke of the piston rod 11, into the pump chamber 12 through a communication pipe 19, through an oil passage 20 formed within the pump rod 9, and further through the inlet valve 6, respectively.
On the other hand, in a compression stroke which follows, the oil present in the pump chamber 12 is pressed by the pump rod 9 and flows into the upper oil chamber 13 through the outlet valve 7 and further through the oil passage 17 formed between the pump cylinder 10 and the piston rod 11.
Then, the oil present in the upper oil chamber 13 flows, in an amount corresponding to the volume of the oil which has thus entered the chamber 13, into the high pressure chamber 18 defined by the diaphragm 4 through a small hole 21 formed in the top of the cylinder 15 and further through an oil passage 22 formed along the outer periphery of the cylinder 15.
Consequently, the gas chamber 5 is compressed in proportion to the volume of the oil which has flowed into the high pressure chamber 18, and the chamber 18 becomes high in pressure, so that the repulsive force of the piston rod 11 increases, causing the vehicular height to increase.
On the other hand, the increase in pressure of the upper and lower oil chambers 13,13 and the resulting increase of the vehicular height cause the recess 8 to appear outside the pump cylinder 10, providing communication between the upper oil chamber 13 and the pump chamber 12, with the result that the pump chamber 12 becomes high in pressure.
Consequently, the inlet valve 6 is forced into a closed state, and this increase of the vehicular height is continued until oil is no longer pumped up into the pump chamber 12 from the oil reservoir 1.
Next, as the vehicular height increases with unloading of goods from the vehicle, the shock absorber stretches to the full length thereof, and when the control orifice 2 appears outside the pump cylinder 10, the upper oil chamber 13 and the oil reservoir 1 are brought into communication with each other through the orifice 2.
Accordingly, the oil present in the upper oil chamber 13 flows into the oil reservoir 1, and the capacity of the gas chamber 5 expands in proportion to the volume of the oil which has left the upper oil chamber 13.
As a result, the upper and lower oil chambers 13,13 decrease in pressure and the piston rod 11 becomes less repulsive, thus causing the vehicular height to decrease.
This decrease of the vehicular height continues until the control orifice 2 assumes a position inside the pump cylinder 10 to cut off the flow of oil between the upper oil chamber 13 and the oil reservoir 1.
In the conventional vehicular height control device, since it is constructed as above, a high pressure receiving rod area is equal to the sectional area of the piston rod 11 when the pump chamber 12 communicates with the upper oil chamber 13 through the recess 8, while when both chambers 12 and 13 are not in communication with each other, the high pressure receiving rod area is equal to an area obtained by substracting the sectional area of the pump rod 9 from the sectional area of the piston rod 11. For this reason the conventional device in question has involved the problem that the repulsive force of the piston rod 11 increases suddenly upon switching from the state of non-communication of both chambers to the state of communication in the expansion stroke and that therefore the vehicle becomes less comfortable to ride in and unstable in its steering performance.
In the pumping operation, when oil is sucked from the oil reservoir 1 into the pump chamber 12 in the expansion stroke, the high pressure receiving area is equal to an area obtained by subtracting the sectional area of the pump rod 9 from the sectional area of the piston rod 11.
On the other hand, in the compression stroke in which the oil present in the pump chamber 12 is forced out into the upper oil chamber 13, the high pressure receiving area is equal to the sectional area of the piston rod 11, so that upon switching to the compression stroke the pressure receiving area increases, causing a sudden increase in the repulsive force of the piston rod. This suddenly increased repulsive force acts like friction and thus results in deteriorated comfortableness.